Why did polyphemus roll back the stone on the morning that the men escaped?

Polyphemus rolled back the stone to let his sheep out for grazing. He was unaware that Odysseus and his men had escaped by hiding underneath the sheep, as he was blind and felt only the wool of the animals. His routine of tending to the sheep provided the perfect opportunity for the men to slip away unnoticed. This moment illustrates the cleverness of Odysseus in using the giant's own habits against him.